Allele Frequency
Definition
The proportion of a specific allele variant at a given genetic locus in a defined population. Forensic match probability calculations multiply the allele frequencies observed at each locus, using the product rule, to compute the overall profile frequency.
- Locus type
- STR or SNP genetic markers
- Combination rule
- Product rule across independent loci
- Data source
- Population reference databases
- Output used for
- Random match probability
Common questions
Why does the calculation need a population-specific frequency table?+
Allele frequencies vary between ethnic and geographic groups because of different mating histories, so the same profile yields a different match probability depending on which reference population is chosen. Labs typically report the figure for the population most relevant to the case, or the most conservative one.
What happens when an allele has never been observed in the reference database?+
Because a frequency of zero would be statistically indefensible, labs substitute a minimum threshold value derived from sample size and confidence-interval methods rather than reporting the allele as impossible. This avoids overstating the rarity of a genuinely rare but real variant.
Related terms
- Hardy-Weinberg Equilibrium (HWE)
- The expected genotype frequency in a large, randomly mating population with no selection, mutation, or migration. Under HWE, the frequency of a...
- Minimum Allele Frequency (Minimum Frequency Ceiling)
- A conservative lower bound applied when an allele is absent or very rare in the database, to prevent an artificially inflated match...
- Population Substructure
- The condition in which allele frequencies differ systematically between subgroups within a nominally single population. Substructure means that individuals within a subgroup...
- Reference Population Database
- A curated collection of allele frequencies or feature measurements from a defined population, used to estimate the probability that a randomly chosen...
- Theta Correction (FST or Coancestry Coefficient)
- A statistical adjustment applied to DNA match probability calculations to account for expected allele-sharing within subpopulations due to substructure. A theta value...